Shofar is one of the very few mitzvos that emphasize listening
לשמוע קול rather than performance of the mitzvah.
When Hashem hears the shofar, it “reminds” Him of one of the high points in His relationship with Israel: the pledge of “na’aseh v’nishma, We will do and we will hear.” Tragically, with the sin of the golden calf we forfeited our claim to the doing part of this commitment (because we did the antithesis of Hashem’s Will), and ever since, the road to Israel’s restoration lies in listening, the nishma, as the Midrash states (Shemos Rabbah 27)
קלקלתם נעשה הזהרו בנשמע you have failed in your pledge to do; be careful to observe your pledge to listen.
Source: Days of Awe – ArtScroll